Mixing of arsenic-rich groundwater and surface water in drinking water distribution systems: Implications for contaminants, disinfection byproducts and organic components

The utilization of groundwaters containing high levels of arsenic (As) for drinking water purposes presents major health and economic challenges for water utilities. One low-cost approach is to mix arsenic-rich groundwater (GW) with arsenic-free surface waters (SW) to achieve acceptable As levels. In this study we investigated the effect of different mixing ratios on water quality in an eastern Croatian water distribution system (WDS). To investigate the effects of mixing on drinking water quality, we measured the organic matter (OM) composition, disinfection byproduct (DBP) and metal concentrations in differently mixed ratios of GW and SW within the WDS. Fluorescence analysis revealed that the GW and SW had similar OM composition, with an almost equal ratio of humic- and protein-like OM throughout the WDS despite fluorescence indices revealing slightly different OM sources between the two water types. The tyrosine-like OM component was more variable, increasing during warmer months and towards the end of the WDS, most likely due to enhanced biofilm formation. Arsenic concentrations decreased to below 10 μg/L in the second half of the sampling campaign. Acceptable water quality was achieved after a period of destabilization and solubilization of loose deposits within the WDS resulting in their mobilization caused by water quality changes. Principal component and classification analysis, regression models and Spearman correlation coefficients revealed an association between As, OM and DBP concentrations with these correlations suggestive of their role in As mobilization in the WDS. Changing source waters, with different OM content and characteristics, corresponded to variable As release within the WDS.

Effects of changing supply water quality on drinking water distribution networks: Changes in NOM optical properties, disinfection byproduct formation, and Mn deposition and release

Field studies were conducted in a Croatian city supplied by two distinct groundwater sources (referred to as A and B) to investigate both the effects of changing water source on the water quality in the drinking water supply system, as well as to further understand discoloration events that occurred in city locations that switched water from source A to B. The water treatment processes at site A were found to alter organic matter (OM) characteristics, removing humic substances while enhancing protein-derived (tryptophan) content. Although the humic-like component predominated in raw waters, microbially/protein-derived components were found to increase throughout the distribution networks of both systems. Disinfection byproducts (DBPs) such as total trihalomethane (TTHM) and total haloacetic acid (THAA) were prevalent in water distribution system (WDS)-A, which correlated with elevated OM content as well as re-chlorination with hypochlorite (NaOCl). Our field study revealed that THMs were more readily formed than HAAs during ClO₂ treatment. Unsurprisingly, chlorite concentrations were generally higher than chlorate concentrations during ClO₂ treatment, whereas (secondary) NaOCl disinfection contributed to higher chlorate production. Principal component analysis indicated that variable pH values and humic-like OM could affect Mn, As and Al concentrations at the consumer's tap. Our results suggested that although Mn concentrations complied with regulations at WDS-B and were below 50 μg/L after disinfection, Mn was oxidized and formed particulate Mn oxides capable of causing discoloration events depending on prevailing network physico-chemical and hydraulic conditions. Aluminium also appears to be released during hydraulic disturbances from extensive deposits within the network. Thermodynamic calculations showed that Mn-oxidation was strongly dependent upon the ORP, and to lesser extent the pH value. Collectively, our results confirm that ensuring the provision of safe drinking waters to consumers requires an understanding of water quality across entire distribution networks in addition to any routine post-treatment monitoring.

Effects of molecular composition of natural organic matter on ferric iron complexation at circumneutral pH

Thermodynamic and kinetic parameters for ferric iron (Fe[III]) complexation by well-characterized humic substances (HS) from various origins were determined by a competitive ligand method with 5-sulfosalicylic acid at circumneutral pH (6.0-8.0) and an ionic strength of ∼0.06 M. The measured Fe binding properties including conditional stability constants and complexation capacities ranged over more than 2 orders of magnitude, depending on the origin and the particular operationally defined fraction of HS examined. Statistical comparison of the complexation parameters to a range of chemical properties of the HS indicated a strong positive correlation between Fe(III) complexation capacity and aromatic carbon content in the HS at all pHs examined. In contrast, the complexation capacity was determined to be up to a few orders of magnitude smaller than the concentration of carboxylic and phenolic groups present. Therefore, specific functional groups including those resident in the proximity of aromatic structures within the HS are likely preferable for Fe(III) coordination under the conditions examined. Overall, our results suggest that the concentration of dissolved Fe(III) complexes in natural waters is substantially influenced by variation in HS characteristics in addition to other well-known factors such as HS concentration and nature and concentration of competing cations present.

Effect of natural organic matter on iron uptake by the freshwater cyanobacterium Microcystis aeruginosa

The mode of Fe uptake by the cyanobacterium Microcystis aeruginosa cultured in Fraquil* (pH 8) containing Suwannee River fulvic acid (SRFA) was examined using short-term radiolabeled (55)Fe uptake assays and a kinetic model that describes extracellular Fe transformations. Both Fe(II) and Fe(III) uptake rates decreased substantially with increasing SRFA concentration as the availability of unchelated Fe decreased due to complexation by SRFA. Fe uptake rates under illuminated conditions were comparable to or slightly higher than those observed in the dark at the same Fe:SRFA concentration ratio, in contrast to results for systems containing ethylenediaminetetraacetic acid where Fe uptake rates were much greater under illumination than in the dark. The limited effect of light principally resulted from the relatively high rates of thermal dissociation and dark reduction of Fe(III) bound to SRFA and complexation of photogenerated Fe(II) by SRFA. Our findings imply that Fe uptake by M. aeruginosa at a fixed total Fe concentration of 200 nM is close to saturation when fulvic acid is present at concentrations near those typically found in natural waters (< ∼5 mg·L(-1)), with cellular growth likely to be limited by Fe availability only when natural organic matter is present at very high concentrations (>25 mg·L(-1)).

Effect of light on iron uptake by the freshwater cyanobacterium Microcystis aeruginosa

Visible light was observed to induce reductive dissociation of organically complexed Fe and dramatically increase the short-term uptake rate of radiolabeled Fe by Microcystis aeruginosa PCC7806 in Fraquil* medium buffered by a single metal chelator, ethylenediaminetetraacetic acid (EDTA). Only wavelengths <500 nm activated Fe uptake indicating that Fe photochemistry rather than biological factors is responsible for the facilitated uptake. The measured rate of photochemical Fe(II) production combined with a significant decrease in (55)Fe uptake rate in the presence of ferrozine (a strong ferrous iron chelator) confirmed that photogenerated unchelated Fe(II) was the major form of Fe taken up by M. aeruginosa under the conditions examined. Mathematical modeling based on unchelated Fe(II) uptake by concentration gradient dependent passive diffusion of Fe(II) through nonspecific transmembrane channels (porins) could account for the magnitude of Fe uptake and a variety of other observations such as the effect of competing ligands on Fe uptake. Steady-state uptake rates indicated that M. aeruginosa acquires Fe predominantly during the light cycle. This study confirms that Fe photochemistry has a dominant impact on Fe acquisition and growth by M. aeruginosa in EDTA-buffered culture medium.

A rapid luminescent assay for measuring cytochrome P450 activity in individual larval Culex pipiens complex mosquitoes (Diptera: Culicidae)

Multiple assays are available to measure P450 activity in insects, including mosquitoes; however, each of these assays has drawbacks in terms of the number of mosquitoes required, specificity, sensitivity, cost, and/or time required to prepare active enzyme homogenates. In this study, a commercially available luminescent assay, P450-Glo, was modified and evaluated to measure P450 activity from the gut of a single larva after removal of the gut contents. We also compared this assay to an earlier developed fluorescent assay. After optimization of assay conditions, the P450-Glo assay held considerable promise to be used as an effective, inexpensive, high-throughput, and sensitive screening assay to measure P450 activities in single mosquitoes. Furthermore, we tested the utility of the single gut assay using the pyrethroid resistant Marin strain of Culex pipiens pipiens form molestus and the pyrethroid sensitive CQ-1 strain of Cx. pipiens quinqefasciatus. We observed on average 1.8-fold higher levels of P450 activity in the resistant mosquitoes in comparison to the sensitive mosquitoes. Additionally, consistent with our previous findings, distribution plots of P450 activity showed 33% of individual Marin mosquitoes had higher P450 activities than the highest activity displayed by a CQ-1 mosquito. The assay platform is highly flexible in terms of choice of tissue, method of preparation, isozyme specificity, and sample quantity and thus could easily be adapted to be used for other arthropod species.

The role of aqueous iron(II) and manganese(II) in sub-aqueous active barrier systems containing natural clinoptilolite

Increasing attention is being placed on capping as a relatively new option in managing both contaminated sediments and dredged materials, due to its economic and environmental benefits. Capping denotes the placement of a cover onto potentially hazardous sediments or dredged material dumps to inhibit the transfer of contaminants into the water column. Retention of divalent iron and manganese cations using sandy capping layers containing natural zeolites as a reactive additive (active barrier systems, ABS) is evaluated in this study. Three different natural zeolite (clinoptilolite) rocks, two from deposits in Australia and one from a North-American deposit, were investigated and compared with respect to their mineralogical, physical and chemical properties. In particular, results from batch and column experiments show that ABS based on these materials can efficiently demobilise iron and manganese from percolating, anoxic pore water by cation exchange under favourable conditions. The retention, however, may be reduced strongly where competitive exchange with divalent cations such as calcium prevails or where mobile colloidal pore water constituents such as clay minerals or humic substances bind fractions of the dissolved iron or manganese. Therefore, the potential of ABS as a means for in situ remediation has to be evaluated diligently with particular regard to the pore water composition of the sediment to be capped.

Removal of the natural hormone estrone from aqueous solutions using nanofiltration and reverse osmosis

The ability of a variety of nanofiltration and reverse osmosis membranes to retain the natural hormone estrone are examined here as a function of solution conditions. While size exclusion dominates retention with the tighter membranes, both size exclusion and adsorptive effects appear to be instrumental in maintaining high retention on nanofiltration membranes that otherwise exhibit relatively low ion retentions. These adsorptive effects may be driven by hydrogen bonding between estrone and the membrane. Electrostatic attraction appears to aid retention with an apparent slight decrease in retention at high NaCl concentrations. Deprotonation of estrone leads to a significant decrease in retention, most likely as a result of the effect of strong electrostatic repulsive forces decreasing the proximity of the negatively charged estrone to the negatively charged membrane surface and thus lowering the potential for adsorptive retention. This deprotonation effect is absent for tight RO membranes. The results reported here indicate that while open nanofiltration membranes may be effective in retaining estrone under some conditions, the extent of retention may be very susceptible to maintenance of adsorptive capacity at the membrane surface and depend on solution chemistry.

Degradation of trace contaminants using coupled sonochemistry and Fenton's reagent

The degradations of phenol in air-equilibrated aqueous media were investigated using coupled sonochemistry and Fenton's reagent for a variety of operating conditions. The decomposition yields of phenol (100-500 microM) were substantially enhanced due to the addition of Fenton's reagent (FeSO4 into the solutions irradiated at 608 kHz with 30 W and with reaction temperature 25 +/- 1 degree C. The decomposition process follows a pseudo-first-order reaction kinetics with respect to phenol concentration, and the rate constant of phenol disappearance observed increases by approximately 2-3 fold when FeSO4 was between 400 and 1000 M at pH = 3.5 +/- 0.2 (controlled by phosphate buffer) as a result of Fe(II) reaction with H2O2 enabling further production of additional OH* radicals. The results obtained here also indicate that the decomposition rate of aqueous phenol using coupled ultrasound and Fenton's reagent was strongly dependant on the initial concentration of reactant, the amount of Fe(II) added as well as the pH of solution. The optimal operating conditions for 100-500 microM phenol decomposition in the air-equilibrated aqueous media were obtained when FeSO4 concentration was between 400 and 1,000 microM with pH in the range 3.5-4.2 under ultrasonic irradiation at 608 kHz, 30 W and reaction temperature 25 +/- 1 degree C.

Effect of floc size and structure on biosolids capillary suction time

Both size and structure of wastewater solids (biosolids) have been hypothesised to have an impact on the dewaterability of these solids yet very little data exists to validate this hypothesis or to elucidate the relative effects of size and structure. We have recently undertaken studies in which the size and structure of activated sludge flocs were altered in a controlled manner and the dewaterability of resultant flocs examined using the well established capillary suction time (CST) test. A small angle light scattering method was applied for the determination of activated sludge floc size and structure. The results obtained in this work show that floc structure is a major determinant of capillary suction time. In some instances, flocs have almost double the median (D(4,3)) size but possess similar fractal dimensions and similar CST values.

Multilevel Structure of Sludge Flocs

In this work, the structure of two kaolin sludges and a waste activated sludge are investigated using both light-scattering and free-settling methods. Fractal dimensions estimated by the light-scattering and free-settling techniques (D(S) and D(F) respectively) differ significantly and support the hypothesis that naturally occurring aggregates possess a multilevel structure. A two-level floc structural model comprised of (i) a primary floc (of fractal dimension D(S)) consisting of primary particles and (ii) a secondary floc (of fractal dimension D(F)) consisting of the microflocs is proposed to interpret the experimental findings. The structural changes of sludge flocs before and after cationic flocculation are interpreted using the proposed two-level model.

Dsorption of estrone on nanofiltration and reverse osmosis membranes in water and wastewater treatment

Adsorption of the trace contaminant estrone, a natural hormone and commonly abundant in surface waters and in treated as well as untreated wastewaters, to eight commercial nanofiltration and reverse osmosis membranes was investigated under well defined conditions. Experiments were conducted in stainless steel stirred cells by spiking trace levels (100 ng x L(-1)) of estrone into five different matrices, namely MilliQ water, a bicarbonate solution, synthetic natural waters containing natural organics, and secondary effluent. Results show that estrone is adsorbed to the membranes to varying degrees with extent of adsorption influenced by the feedwater composition with different mechanisms of association controlling adsorption to different membrane types. Increase in membrane resistance is typically observed to result in decrease in extent of estrone adsorption.

Chemiluminescence of luminol in the presence of iron(II) and oxygen: oxidation mechanism and implications for its analytical use

The validity of chemiluminescence-based methods relies upon the uniqueness of the relationship between the concentration of the analyte and the intensity of chemiluminescence produced. We have examined the chemiluminescence of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) in the presence of O2, without added H202, to measure nanomolar concentrations of total Fe(II) (both inorganically and organically complexed) in aqueous samples. To test the validity of the method, we have developed a kinetic model that describes the two-step oxidation of luminol by superoxide and hydroxyl (or hydroxyl-like) radicals produced from the oxidation of Fe-(II) by O2 by synthesis of existing published data. This model was coupled with a model for Fe(II) oxidation by O2 in the presence and absence of the naturally occurring Suwannee River fulvic acid (SRFA). The production of chemiluminescence depended upon the concentrations of free radicals in both the sample and luminol reagent and the pH at which the reactions were performed. The relationship between Fe(II) concentration and chemiluminescence intensity was found to be unique at Fe(II) concentrations from 1 nM to 1 microM without organic complexation and from 1 to 32 nM in the presence of SRFA despite strong signal quenching in the latter case. This type of behavior will likely ensure a similarly unique relationship in the presence of a wide range of organic compounds; however, when other systems are being investigated, the technique should be carefully evaluated.

Cost factors and chemical pretreatment effects in the membrane filtration of waters containing natural organic matter

This paper compares the membrane processes available for water treatment. Membranes have the advantage of currently decreasing capital cost, a relatively small footprint compared to conventional treatment, generally a reduction in chemicals usage and comparably low maintenance requirements. Three membrane processes applicable to water treatment, micro- (MF), ultra- (UF), and nanofiltration (NF), are compared in terms of intrinsic rejection, variation of rejection due to membrane fouling and increase in rejection by ferric chloride pretreatment. Twelve different membranes are compared on the basis of their membrane pore size which was calculated from their molecular weight cut-off. A pore size of < 6 nm is required to achieve substantial (> 50%) organics removal. For a fouled membrane this pore size is about 11 nm. UV rejection is higher than DOC rejection. Coagulation pretreatment allows a higher rejection of organics by MF and UF and the cut-off criterion due to initial membrane pore size is no longer valid. A water quality parameter (WQP) is introduced which describes the product water quality achieved as a function of colloid, DOC and cation rejection. The relationship between log (pore size) and WQP is linear. Estimation of membrane costs as a function of WQP suggests that open UF is superior to MF (similar cost at higher WQP) and NF is superior to tight UF. Chemical pretreatment could compensate for the difference between MF and UF. However, when considering chemicals and energy costs, it appears that a process operated at a higher energy is cheaper at a guaranteed product quality (less dependent on organic type). This argument is further supported by environmental issues of chemicals usage, as energy may be provided from renewable sources.

Effect of Fe(III)-ligand properties on effectiveness of modified photo-Fenton processes

This paper examines a modified photo-Fenton (UV/Fe oxalate/H2O2) process. The degradation of oxalate in this system in the absence of Reactive Red 235 was studied using both experimentation and kinetic modelling. The degradation of Reactive Red 235 in this system was also studied. Light intensity and solution pH had large effects on the degradation of both oxalate and Reactive Red 235, with the effect of pH not due simply to speciation changes. The most important properties of the oxalate ligand in the UV/Fe oxalate/H2O2 process are that it forms Fe(III)-oxalato complexes that are easily photolysed and also is relatively unreactive with OH. radicals.

Comparison of laboratory uranium sorption data with 'in situ distribution coefficients' at the Koongarra uranium deposit, Northern Australia

Distribution coefficients derived from laboratory sorption experiments are commonly used to model the migration of long-lived radionuclides in the environment. However, it has been suggested that field measurements in natural systems ('in situ distribution coefficients') may provide a more accurate indication of 'true' partitioning coefficients than laboratory experiments. In this paper, the relationship between field and laboratory sorption data for uranium is evaluated, using data from the Koongarra uranium deposit in Northern Australia. An extensive suite of laboratory sorption measurements and in situ partitioning data for U has been obtained at this site. A valid comparison can only be made when the calculation of field partitioning is based on U in 'accessible' phases (rather than total U in the solid) and U species in true solution (i.e. excluding particles). In this study, accessible U was estimated using a chemical extraction and the results were verified using an isotope exchange technique. A satisfactory correspondence between field and laboratory partitioning data was obtained when the pH values and partial pressures of CO2 in laboratory sorption experiments were similar to those found in the field. Under these conditions, the measured laboratory sorption ratios (Rd) and in-field partitioning values (Pacc) for U at Koongarra were in the range between approximately 1 x 10(3) and 2 x 10(4) ml/g. However, the distribution of U in solid and groundwater phases at Koongarra is extremely heterogeneous. This variability must be taken into account when modelling radionuclide migration at this site.

Size and structure effects on centrifugal dewatering of digested sewage sludge

The application of light scattering over small angles for the determination of digested sludge floc size and structure and its relationship with dewaterability is investigated. It appears that improved dewatering corresponds with lower floc fractal dimension (a more open structure) and a smaller proportion of fine particles. The initial increase in fractal dimension with increasing polymer dose for the digested sludge is most likely due to more efficient aggregation of the finer particles and the resulting formation of denser particle aggregates. A large colloidal fractal of the digested sludge (< 10 microm) appears to be less negative than the bulk digested sludge. This suggests that the fine particles will react differently and possibly less aggressively to the cationic polymer than the larger and more negative particles. The higher negative charge associated with the larger particles might be related to greater levels of highly negatively charged extracellular polysaccharides (EPS) adsorbed to the flocs or could result from the association of FeS with the finer fraction. The appearance of much greater levels of fine particles after digestion suggests that the flocs have undergone disintegration. Whether this is due to reduced biological efficiency within the digestor or iron reduction under the anaerobic conditions is not known for certain, although no indication of prolonged stress in the digesters could be found from plant performance data.

Effect of aggregate size and structure on specific resistance of biosolids filter cakes

Investigation of the specific resistance to filtration of biosolids modified by cationic polymer addition reveals a dependence on both size and structure of the resulting biosolids flocs. This dependence is reasonably described by a modified form of the Carman-Kozeny equation and reveals that effects of differences in floc structure on cake resistance are most marked for flocs of small size. Conversely, effects of differences in floc size are most significant when flocs are more compact.

Structure of Hydrous Ferric Oxide Aggregates

The small angle light scattering behavior of hydrous ferric oxide flocs is examined here and found to provide useful insights into the nature of the aggregates formed despite the large size of these aggregates at later times. The flocs appear to exhibit fractal properties over a significant size range though the aggregates appear to be easily disrupted through mixing effects resulting in breakup and/or restructuring to denser assemblages. Background electrolyte concentrations also have some impact on floc structure but mixing effects and apparent destabilization by ferric ions limit the effect of added electrolytes on the stability and structure of ferric oxyhydroxides. Similar estimates of fractal dimensions of these hydrous ferric oxide flocs are obtained both by static light scattering analysis and by a cluster mass scaling approach. The choice of density distribution cutoff function has some impact on derived size and structure parameters and further refinement in this area is needed. Copyright 2000 Academic Press.

Colloidal Fouling of Ultrafiltration Membranes: Impact of Aggregate Structure and Size

A close coupling between the structure and size of hematite flocs formed in suspension and the permeability of the cake that accumulates on ultrafiltration membranes is observed. Specific resistances of cakes formed from flocs generated under diffusion-limited aggregation conditions are at least an order of magnitude lower than those of cakes formed from flocs generated under reaction-limited aggregation conditions. Similar effects are observed whether the aggregation regime is controlled by salt concentration, pH, or added organic anions. This dramatic difference in cake resistance is considered to arise from the size and fractal properties of the hematite assemblages. The ease of fluid flow through these assemblages will be influenced both by the fractal dimension of the aggregates and by their size relative to primary particle size (since, for fractal aggregates, porosity increases as the size of the aggregate increases). The size and strength of aggregates are also important determinants of the relative effects of permeation drag, shear-induced diffusion, and inertial lift and result, in the studies reported here, in relatively similar rates of particle deposition for both rapidly and slowly formed aggregates. The results presented here suggest that control of cake permeability (and mass) via control of aggregate size and structure is an area with scope for further development though the nature and extent of compaction effects in modifying the fractal properties of aggregates generated in suspension requires attention. Copyright 1999 Academic Press.

Light-dependent reduction of copper(II) and its effect on cell-mediated, thiol-dependent superoxide production

The thiol-dependent reduction of copper(II) and associated superoxide radical production in copper(II)-treated cultures of the unicellular alga Dunaliella tertiolecta is induced by light of wavelength greater than 425 nm. We propose that the effect of light on these reactions is mediated at the copper(II) reduction step through a light-stimulated ligand to metal charge transfer reaction between sulphur and copper(II). These results suggest that light may be an important controlling factor in metal-induced lipid peroxidation reactions.